One type of prior art clean air filtration system employs an ionizer to create ions which attach themselves to dust and dirt particles. The charged particles are then collected such as in a filter or an electrostatic precipitator. The efficiency of such a system depends heavily on the effectiveness of the ionizer to create charged particles which can then be collected.
Traditionally, two types of ionizers have been used in clean air filtration systems (room purifiers) to enhance the performance of the filter used to collect dust and dirt particles.
One type of ionizer consists of a plurality of wires and ground plates. When a high voltage is applied to the plurality of wires, the electric field created between the wires and plates breaks down air molecules, creating large numbers of ions. The ions move to the ground plates at very high speed and collide with dust and dirt particles in the air, transferring electrostatic charges to the dust and dirt particles. These wire-plate type of ionizers are usually disposed upstream of a filter system to pre-charge dust and dirt particles for collection in the filter system. While an effective mechanism for charging particles, this type of ionizer is expensive to construct, requires a high operating current, making it expensive to operate, and is a potential safety hazard due to the very high voltages and high currents employed. This type of ionizer is commonly used in controlled air spaces such as furnace and air conditioning ducts.
Another type of ionizer, which is widely used in room air cleaners or purifiers, is a point ionizer. In a point ionizer, a high voltage, but a much lower current than is typically used in a wire-plate type ionizer, is applied to a point electrode or electrodes to create ions. Again, these ions charge particles of dust and dirt and thereby enhance the performance of a filter.
It is typical of these cleaners or purifiers for the point ionizers to be positioned at or near the exit of the air passing through the cleaners or purifiers. Typically, this is done to disperse ionized particles throughout the room. At least some of these ionized particles would then find their way back to the inlet of the cleaners or purifiers and aid in the operation of the cleaners or purifiers.
An example of an exit point ionizer is U.S. Pat. No. 4,376,642, Verity, Portable Air Cleaner Unit, which describes a portable air cleaner unit. An air mover such as a fan is disposed downstream of the main filter, and an exposed negative ion source is disposed downstream of the fan on the external surface of the air outlet. The main filter consists of fibers shredded from a non-carcinogenic plastic membrane which has been permanently electrostatically charged. The negative ion source ionizes the cleaned air as it leaves the cabinet.
Another example of an exit point ionizer is U.S. Pat. No. 5,268,009, Thompson et al, Portable Air Filter System, a portable air filter system for use in the home, offices, or other areas where it is desired to remove airborne particulate matter from the air. The air filter system includes an ionizer for supplying negative ions to the air exiting through the outlet. The ions charge foreign particles in the air. As a result, when the charged foreign particles are drawn into the inlet of the system, the particles are retained on the filter medium.
Still another example of an exit point ionizer U.S. Pat. No. 5,332,425, Huang, Air Purifier, which describes an air purifier having an extended and tapered discharging copper needle is electrically coupled to a high voltage generator contained within the purifier housing and produces negative ions. The discharging needle is pointed in contour and has an apex end located adjacent the air exit opening. The discharging needle extends in the direction of the passage of high pressure air from the purifier housing which allows the discharging needle to vibrate responsive to the high pressure air flow and increases the amount of negative ions mixed with the air passing from the purifier housing.
These exit ionizers are very effective at charging particles, and has much lower cost and little safety hazard. However, point ionizer systems typically are positioned at the air exit of the purifier, i.e., downstream of the filter. With exit air ionizers, charged particles are discharged into room air, and stay in the air for a significant amount of time before being re-circulated through the filter. As a result, a significant number of these charged particles are removed by other external surfaces such as walls, carpets, human bodies and furniture surfaces, instead of the filter.
Other ionizing filtration systems use point source ionizers at or near the air inlet to the filtration system. Typically, these filtration systems are designed to either disperse ions throughout the room, as do exit ionizers, or are designed to inject ions directly into the air stream within the air inlet of the filtration system.
An example of the type of air cleaning apparatus which diffuses ions throughout the room is shown in U.S. Pat. No. 5,980,614, Loreth et al, Air Cleaning Apparatus, which describes an air cleaning apparatus, especially for cleaning of room air. The device includes an ionizing device having a unipolar ion source formed by a corona discharge electrode, an electrostatic precipitator connected to a high-voltage source and having a flow-through passageway for air to be cleaned and two groups of electrode elements of one group being interleaved with and spaced from the electrode elements of the other group and arranged to be a potential different from that of the other group. While the corona discharge electrode is positioned near the air inlet to the apparatus, the corona discharge electrode is arranged such that the ions generated at the electrode can diffuse essentially freely away from the electrode and thereby diffuse substantially freely throughout the room in which the ionizing device is positioned. As such, the apparatus described in Loreth et al suffers from many of the same disadvantages as the exit ionizers discussed above.
Air filtration systems which are designed to inject ions directly into the air stream at or near the air inlet of the air filtration system or with the internal air stream of the filtration system typically do not achieve optimum efficiency in air cleaning. Typically, in these systems the number of ions generated and the ability of the ions generated to attach to particles of dust and dirt are limited both by the proximity of the ion generation source to the ion collector and by the limited length of time in which the ions have to attach to particles of dust and dirt in the air flow stream within the filtration system.
Thus, while many prior art systems exist which utilize ion generators, and which utilize point source ionizers, such prior art systems suffer numerous disadvantages as discussed above.
Some prior art air filtration systems utilize a centrifugal fan to move air through the filtration system. While such fans are efficient and are operational over a wide range of pressure drops, centrifugal fans are relatively noisy. As such, centrifugal fans suffer significant disadvantages for use in portable, room air filtration systems. Axial fans are considerably less noisy, deliver a uniform straight airflow and can be made very small but are very sensitive to pressure drops as such their use in filtration systems is limited.